The aims of this study are to 1) characterize reactivity of the skin (i.e., mast cells) to injection of synthetic C3a peptides in normal human subjects, and in people whose skin mast cells may be "nonspecific" releasers of histamine. In particular, people who have had an adverse reaction to intravenous injection of radiographic contrast media (RCM) will be examined; 2) compare skin reactivity to synthetic C3a peptides injected in "normal" subjects to reactions in other groups of patients susceptible to hives, including atopic patients and patients with chronic idiopathic urticaria and angioedema; and 3) develop a screening assay which would reliably predict patients at greatest risk of adverse reactions to RCM or other stimuli.